A superconducting point contact is used to determine the spin polarization at the Fermi energy of several metals. Because the process of supercurrent conversion at a superconductor-metal interface (Andreev reflection) is limited by the minority spin population near the Fermi surface, the differential conductance of the point contact can reveal the spin polarization of the metal. This technique has been applied to a variety of metals where the spin polarization ranges from 35 to 90 percent: Ni0.8Fe0.2, Ni, Co, Fe, NiMnSb, La0.7Sr0.3MnO3, and CrO2.

Using the point contact Andreev reflection technique, we have carried out a systematic study of the spin polarization in the colossal magnetoresistive manganite, La0.7Sr0.3MnO3 (LSMO). Surprisingly, we observed a significant increase in the current spin polarization with the residual resistivity. This counterintuitive trend can be understood as a transition from ballistic to diffusive transport in the contact. Our results strongly suggest that LSMO does have minority spin states at the Fermi level. However, since its current spin polarization is much higher than that of the density of states, this material can mimic the behavior of a true half-metal in transport experiments. Based on our results we call this material a transport half-metal.A half-metallic ferromagnet is a metal that has an energy gap at the Fermi level, E F , in one of the two spin channels. Only the other channel has states available for transport, and thus the electric current is fully spin-polarized. Finding half-metallic or other highly spin-polarized metals would bring about major advances in magnetoelectronics, since device performance improves dramatically as the spin polarization of the metal approaches 100%.1 Although half-metallicity has been predicted in quite a number of materials, the experimental situation is still controversial, especially for the manganese perovskite, La 0.7 Sr 0.3 MnO 3 (LSMO). Theoretical 2 and experimental values 3-6 of the spin polarization of this fascinating material with highly unusual structural, magnetic and electronic properties, obtained by different techniques vary from 35% to 100%. Not surprisingly, when Park et al. concluded from their spin-resolved photoemission spectroscopy measurement that LSMO is completely spin-polarized 3 it attracted immediate attention. This result was important not only from a practical viewpoint, but also as a potential new insight into the microscopic physics of this system, since the values of the spin polarization are extremely sensitive to the band structure of LSMO. Importantly, the measured value of the spin polarization, P n , depends on the experimental technique. It is often possible 10 to define P n in the following form:where N ↑ (E F ), N ↓ (E F ) and v F ↑ , v F ↓ are the majority and minority spin DOS and the Fermi velocities, respectively. This definition allows a direct comparison between different experiments and the theory, since all the quantities in Eq. 1 can be evaluated from the band structure. The spin polarization P 0 (n =0) measured by spinresolved photoemission measurements is determined only by the DOS at the Fermi level.11 Transport experiments measure a different spin polarization, which includes the Fermi velocities (Eq.1). In the ballistic, or Sharvin, limit (mean free path, L, larger than the contact size, d) the DOS is weighted linearly with v F , and P 1 is measured. 12In the diffusive, or Maxwell regime (L < d), as in the classical Bloch-Bolzmann theory of transport in metals, the weighting is quadratic in v F (n=2) and P 2 is measured (a...

We have developed a simple method to measure the transport spin polarization of ferromagnetic materials. This technique relies on the fact that the Andreev reflection process at the interface between a superconductive and normal is influenced by the spin polarization P of the normal metal. In a very short time we have been able to measure the spin polarization of several metals: NixFe1−x, Ni, Co, Fe, NiMnSb, La0.7Sr0.3MnO3, and CrO2, whose spin polarization ranges from 35% to 90%. Our results compare well with other methods for measuring P.

Highly spin-polarized chromium dioxide (CrO2) thin films were deposited on (100) TiO2 substrates by chemical vapor deposition using chromyl chloride as a precursor. The spin polarization, as measured by the point contact Andreev reflection technique, was 81±3%. X-ray diffraction θ/2θ scans indicated the films grew completely (100) oriented, in registry with the (100) oriented TiO2 substrate. X-ray diffraction φ scans on the CrO2 (110) reflection indicated the expected twofold symmetry, with no evidence of misaligned material. The resistivity at room temperature was 240 μΩ cm and decreased to 10 μΩ cm at 5 K, consistent with metallic behavior. The films were ferromagnetic with a Curie temperature of 395 K and a coercivity of ∼100 Oe at 298 K. The use of chromyl chloride as a precursor resulted in efficient and controlled CrO2 film growth.

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It is demonstrated here that granular films of Y-Ba-Cu-O may serve as optical detectors, operating at wavelengths from the visible to the far infrared, at temperatures well above that of liquid helium. Preliminary measurements using a blackbody source show that an upper bound of the minimum detectable power is 1 μW. The response time as determined by a pulsed far-infrared source is of the order 20 ns. Methods to improve the sensitivity will be discussed.

Nb-Ta multilayered films prepared by rnagnetron sputtering have been studied by critical-field measurements.We have examined the effects of substrate orientation and deposition temperature on the properties of the films. Three-dimensional to two-dimensional crossover is observed. For films with larger Nb layer thicknesses an additional transition in H, &~I at lower temperatures is observed which cannot be accounted for by the interfacial regions.Since the original work on Nb-Ta multilayers by Durbin et al. , ' it has been recognized that this system is the closest realization of a perfect metallic superlattice. Outside of the tunneling studies of Hertel et al. , very little work has been done on the superconducting properties of the system. Encouraged by the work of and Nb-Ti (Ref. 5), we have undertaken a study of the critical fields of the Nb-Ta system in order to examine the . . . S-S'-S-S'. . . multilayer where S and S' denote superconductors.We have measured perpendicular and parallel critical fields and their angular dependence for several Nb-Ta multilayers. In addition we examine our results in light of the theoretical analysis of Biagi et al.

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